How to group by tables in datatable and sum them using ER

I am a newbie here and I have an issue: I have 5 tables in my database with the same columns and headers, so I want to group by all the tables and I want to sum them.

Suppose if there is a column Amount which is present in all 5 tables, then I want to group by the Amount and I want to sum all the 5 table values using Entity Framework.

This is my code which I'm using to do group by, but I'm unable to do the sum:

private DataTable ProcessData(DataSet ds)
{
DataTable dt = new DataTable();
dt = ds.Tables[0].AsEnumerable()
.GroupBy(r => new { Col1 = r["Name"], Col2 = r["Amt"], Col3 = r["Net"], Col4 = r["Data"] })
.Select(g => g.OrderBy(r => r["max"]).First())
.CopyToDataTable();
return dt;
}